1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate (e.g., a workpiece, an object, a display, etc.). The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern. This is referred to as maskless lithography.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
Another way of imaging includes pixel grid imaging, in which a pattern is realized by successive exposure of spots.
It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. Instead of a mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements. An advantage of such a system compared to a mask-based system is that the pattern can be changed more quickly and for less cost.
Many patterning devices are binary, that is the dose delivered to a given point on the substrate can be one of two kinds, a high level or a low level. Normally the high level is set at a level that will expose the resist, i.e., is above the resist threshold, whereas the low level is below the resist threshold and does not expose the resist.
Gray scaling is, however, only directly achievable with some patterning devices, for example those which operate as a grating light valve. These devices only have a limited number of pixels, reducing their usefulness. For other devices, a gray scale can be obtained by switching an individually controllable element on and off at a controlled rate. This has the disadvantage that it decreases the throughput at which the apparatus can operate.
What are needed, therefore, are improved gray scaling imaging methods and systems.